Acidic graphene organocatalyst for the superior transformation of wastes into high-added-value chemicals

Our dependence on finite fossil fuels and the insecure energy supply chains have stimulated intensive research for sustainable technologies. Upcycling glycerol, produced from biomass fermentation and as a biodiesel formation byproduct, can substantially contribute in circular carbon economy. Here, we report glycerol’s solvent-free and room-temperature conversion to high-added-value chemicals via a reusable graphene catalyst (G-ASA), functionalized with a natural amino acid (taurine). Theoretical studies unveil that the superior performance of the catalyst (surpassing even homogeneous, industrial catalysts) is associated with the dual role of the covalently linked taurine, boosting the catalyst’s acidity and affinity for the reactants. Unlike previous catalysts, G-ASA exhibits excellent activity (7508 mmol g−1 h−1) and selectivity (99.9%) for glycerol conversion to solketal, an additive for improving fuels’ quality and a precursor of commodity and fine chemicals. Notably, the catalyst is also particularly active in converting oils to biodiesel, demonstrating its general applicability.

The authors report their findings on a functionalised graphene catalyst which has been applied to the formation of solketal from glycerol and acetone. The functionalised catalyst has a high activity compared to many other catalysts, helpfully as specific activity as this is not always given in the field, both homogeneous and heterogeneous and as such represents a worthy advance in production of a useful molecule from a bio-sourced waste. Further, the catalyst can be recycled in a simple manner with little to no loss in activity, a point about which is a useful parameter to allow comparison to other recyclable catalysts. Overall, the manuscript is well presented and DFT calculation included along with key characterisation has been reported, both pre-and post-reaction. I would like to know if the taurine is stable over the reaction period and did the authors note any S present in the liquid fraction postcentrifugation? Furthermore, was the reaction monitored over the 1 h time period, is there any significant change in selectivity or reduction in reaction rate over the typical reaction profile seen whereby the rate reduces as the reactants are used up? The authors apply the catalyst to another process and highlight its efficacy and helpfully compare the ester yield to other similar catalysts. Clearly, this should inspire other groups to consider this type of material. However, two points strike me as being a challenge when discussing new approaches to industrial application. Complicated catalyst preparations and use of crude feed-stocks. The catalyst is an order greater in activity to H2SO4 and recyclable without the side-waste described by the authors. This element is not a large hurdle and would be worthwhile to pursue following some analysis of the potential waste vs productivity and economics. The later point is perhaps more pertinent, in that typically glycerol is not available cheaply in a purified form. The biodiesel industry produces a very mixed glycerol waste stream. Tolerance of those additives would be a clear advantage and perhaps worth studying in the future. Could the authors comment on the general robustness of the catalyst, TGA experiments are mentioned and show that the taurine groups are removed >200C which suggests good adhesion? Perhaps this relates to the above points which were not addressed with respect to time-online profiles and potential desorption of taurine during reaction. I fully appreciate that the catalyst appears recyclable, however, perhaps the taurine density is such that it can afford to lose an appreciable quantity of active sites and maintain good activity. In summary, I recommend publication following addressing these minor points. Minor issue, line 226 Table S2 should be Table S1?
Reviewer #3 (Remarks to the Author): Poulose et al. performed glycerol acetalization reaction using amino acid functionalized graphene catalyst at ambient conditions. Key points are amino acid functionalization, solvent-free reaction conditions, high specific productivity, and theoretical evidence to explain the catalytic activity. It deserves to publish after major revisions, and following issues should be addressed: 1. In Section 4.2., authors have mentioned "Finally, the dispersion was acidified by 25 wt.% sulfuric acid to secure that all acidic sites are protonated, and finally washed via centrifugation cycles with methanol followed by freeze drying, and this material was used for characterization and further experiments". It is not clear of using H2SO4. If the amino acid functionalized graphene catalyst needs to be activated by H2SO4, then what is the purpose of using taurine? Sulfonic acid functionalized GO or rGO by H2SO4 could be the best choice over the synthesized catalyst. Therefore, authors are requested to compare the catalytic activity with sulfonic acid functionalized GO or rGO. 2. During final dispersion of the catalyst with 25 wt.% sulfuric acid, whether any oxygen and sulfonic acid functionalities incorporate at the edge of the graphene or not need to be confirmed by CHNS/O analysis. In XPS, -SO4 species is also found. How it is attached with the catalyst and has it any role in this reaction? 3. In C (1s) XPS spectrum, the peak corresponds to B.E. approx. 290 eV is for C-F or π-π* shake-up satellite peak? As authors stated that almost all F atoms had been eliminated. 4. The authors are asked to make the S (2p) XPS B.E. range uniform for main text and SI. 5. In section 2.2. and Figure 3, authors claimed that they obtained 96.5 % glycerol conversion and 96.8 % solketal selectivity. Reporting activity to one decimal place will be accurate when error bar will be provided. 6. Why lowering the glycerol: acetone mole ratio improves the catalytic activity? 7. During recyclability test, authors protonated the catalyst with H2SO4. However, in the mechanism -SO3H species remains intact. So, what was the necessity to use H2SO4. The hot filtration test should be performed to check heterogeneity of the catalyst. 8. DFT energies are only accurate when all details will be provided in SI (e.g. frequency, energy, thermal correction, coordinates…).

Reply:
As suggested by the reviewer, we have added the information about the specific productivity (mass-normalized rate of product formation), and its citation (Applied Materials Today 23, 101053, 2021) on page 3. In the experimental section, page 20, it is mentioned that specific productivity is calculated based on the amount (mmol) of product per time unit (h) relative to the mass of the catalyst (g): Specific productivity = ( ) ( ) × ( ) Comment 1.4. When doing deconvolution analysis of the C1s peak, the authors are suggested to consider XPS signal originated from the contamination carbon, usually used to calibrate the others at 284.6 eV, then reconsider the related data, and related analysis and discussion.
Reply: All XPS data are corrected against the C1s 284.6 eV before deconvolution and plotting. In the present case, since the material (the catalyst itself) is carbon/graphene, we have used the aromatic carbon signal set at 284.6 eV as the reference point for the calibration of all other signals.
We have carefully checked all spectra to secure that all calibration and analyses are based on the 284.6 eV peak of aromatic carbon components.
Comment 1.5. In Figure 3, please check if the chemical formula of solketal is correct.

Reply:
We thank the reviewer for identifying this mistake in the chemical formula. There was one extra OH in the solketal formula, which should be only O, and is now corrected (Fig 3a, page 10).
Comment 1.6. The authors are suggested to describe if acetal was also quantified, and how about the carbon balance of the catalytic system.

Reply:
In the revised manuscript, we have calculated the carbon balance for the catalytic reaction after quantifying all the products, including acetal. The reaction was performed at room temperature with 0.5 wt % of catalyst with respect to glycerol (1.0 g). After 1h, methanol was added to the reaction mixture and the catalyst was separated from reaction mixture using centrifuge. The product mixture was analyzed by gas chromatography. The carbon balance, not considering the excess of acetone, but only the stoichiometric amount i.e. the same as glycerol (10.85 mmol), is 98%. Considering the remaining glycerol and acetal as wastes, and solketal as the only carbon-containing product, then the carbon balance is 93.4% Comment 1.7. When testing the reusability of the catalyst, "the sample was protonated by washing with 25 % sulfuric acid and then washed with methanol to remove excess acid" the authors are suggested to explain why washing by sulfuric acid is needed. If there were data without such pretreatment processes, the reusability of the catalyst would be more persuasive.
Reply: This is a very good point raised by the reviewer. In the revised manuscript (page 18), we have clarified this and included the following text, explaining the use of sulfuric acid washing.
"H2SO4 washing was performed to ensure that the taurine molecule's sulfonate group conjugated on the catalyst is protonated, avoiding internal salt formation with the amine group of taurine." The reviewer is correct that since the G-ASA material is a catalyst, after the first treatment for full protonation, there should be no further need to wash the catalyst with H2SO4 after each cycle. Therefore, in the revised manuscript, we also performed the catalyst recycling without H2SO4 washing steps and received the same performance on both conversion and selectivity.
In the experimental part, page 21 we have modified the washing procedure and highlighted the changes as given below: "The product was analyzed by GC, and the used catalyst was recovered by centrifugation and washed with acetone several times to remove the impurities adsorbed on the catalyst. The final precipitate was dried at 60 °C overnight before being used for the next cycle."

Reply:
The reviewer is right that the mechanistic discussion must be based on reliable reaction pathways connecting reactants and products via corresponding transition states (TSs). We paid particular attention to this issue using scans along reaction coordinates, instead of using the IRC approach. Although the IRC approach is a useful tool to investigate reaction pathways, it requires the knowledge of the TS structure including the initial force constants, which then enables to proceed along the direction of the transition vector back and forward towards the reactants and products, respectively. Such approach is appropriate for (rather small) systems, which can be fully relaxed in the geometry optimization steps and for which the evaluation of the Hessian matrix is affordable. It is, however, not suitable for investigating the pathways involving large structures, where the "active region" is attached to a skeleton partially frozen during the optimization step as it is in our case (only the active region was allowed to relax during the optimizations with the carbon lattice kept frozen).
In addition, the modeled catalytic reaction takes place in highly acidic environment, which implies the possibility of proton transfer events, in which the environment can be involved. To localize the TSs in such cases is a challenging task even for much smaller systems. Nevertheless, to explore the reaction pathways and estimate the related barriers, we performed a series of back and forward (partially relaxed) scans starting from the reactants and products (or intermediates) in each reaction step along a carefully chosen reaction coordinate. In such an approach, it is assumed that the structure changes its protonated state (i.e., the proton transfer occurs) if the potential energy curve becomes lower along the particular scan. For example, during the formation of an adduct A without the catalyst ( Figure S6 in SI), the formation of a CO bond between glycerol is accompanied by a proton transfer from glycerol to the oxygen atom of acetone, which can however be assisted by the acidic environment. Therefore, the barrier shown in Fig. 6a is estimated from back and forward scans as shown in Fig. 6b. It should also be underlined that the choice of the internal reaction coordinates is in our case always chemically well founded, because it can be presumed that the formation of adducts (steps 1-3 in Figure 4) involves an attack of one of the oxygen atoms of glycerol on the carbonyl group of acetone and also the formation of cyclic products (steps 4-6 in Figure 4) requires approaching specific atoms.
To make this point clear for the reader, we added a short paragraph to Computational details in SI (page 4). The scans based on which the barriers were determined have been added to Figures S7, S9, and S14.
In addition being inspired by the reviewer`s comment, we performed new calculations for systems allowing to localize the TS structure analytically (e.g., for cyclization of protonated adducts A and B), which confirmed that the barriers determined based on relaxed scans were reasonable (cf. Figure  S11 and new Table S3 shown below).   Implemented changes: The following paragraph was added to Computational details in SI page 5: "It should be emphasized that the modeled catalytic reaction takes place in highly acidic environment, which implies the possibility of proton transfer events, in which the environment can be involved. To estimate the activation barriers, we performed a series of back and forward (partially relaxed) scans starting from the reactants and products (or intermediates) in each reaction step along a carefully chosen reaction coordinate. In such an approach, it was assumed that the structure changed its protonated state (i.e., the proton transfer occurs) if the potential energy curve became lower along the particular scan. For example, during the formation of an adduct A without the catalyst ( Figure S6 in SI), the formation of a CO bond between glycerol was accompanied by a proton transfer from glycerol to the oxygen atom of acetone, which could however be assisted by the acidic environment. Therefore, the barrier shown in Fig. 6a was estimated from back and forward scans as shown in Fig.  6b. Figure 4) required approaching specific atoms."

It should also be underlined that the choice of the internal reaction coordinates was chemically well founded, because it could be presumed that the formation of adducts (steps 1-3 in Figure 4) involved an attack of one of the oxygen atoms of glycerol on the carbonyl group of acetone and also the formation of cyclic products (steps 4-6 in
Figures S7, S9, and S14 were modified to include the scans based on which the barriers were determined: Figure S7. (a) Energy diagram (in kcal/mol) of the first phase of the catalyzed reaction, i.e. the formation of an adduct A (steps 2a and 3a in Figure 5) along the C-65(acetone)O-86(gly) coordinate (see Figure S8)   A new Table S2 was added to SI:  Table S2" should be " Table S1".

Reply:
We thank the reviewer highlighting this aspect. We have carefully reviewed the manuscript before submitting the revised version and have corrected the table numbers and other small mistakes.
Comment 1.10. Lines 296-298, please check if the sentence, "In another work, transesterification of tripalmitin to palmitic acid ester was catalyzed by superhydrophobic mesoporous polymers and obtained a yield of 99.9% after a 16-hour reaction at 65 °C.", needs to be improved.

Reply:
We decided to remove the particular example in the revised manuscript because it is about transesterification (page 16). We kept the other examples that refer to esterification, fully matching with the esterification reaction discussed in this manuscript.

Point-By-Point Answers to the Reviewers' Comments Reviewer #2
The authors report their findings on a functionalised graphene catalyst which has been applied to the formation of solketal from glycerol and acetone. The functionalised catalyst has a high activity compared to many other catalysts, helpfully as specific activity as this is not always given in the field, both homogeneous and heterogeneous and as such represents a worthy advance in production of a useful molecule from a bio-sourced waste. Further, the catalyst can be recycled in a simple manner with little to no loss in activity, a point about which is a useful parameter to allow comparison to other recyclable catalysts. Overall, the manuscript is well presented and DFT calculation included along with key characterisation has been reported, both pre-and post-reaction.
Comment 2.1. I would like to know if the taurine is stable over the reaction period and did the authors note any S present in the liquid fraction post-centrifugation?
Reply: This point is certainly very important. After synthesis of the catalyst, a step involving extensive dialysis removes non-covalently bound taurine molecules from the graphene's surface. Furthermore, the stability of the taurine molecule is evident from the persistence of the sulfur content, according to the XPS spectra, after three (Fig. S3) and five (Fig. S16) recycling reactions, indicating the stability of the catalyst and of the sulfur-containing taurine molecules immobilized on the catalyst. In addition, we have also performed the catalyst leaching test to check and confirm the heterogeneity of the catalyst. According to this, the catalyst was separated and removed from the reaction mixture after 5 min from starting the reaction. The reaction was allowed to continue. However, glycerol and solketal concentrations did not changed further, indicating that the G-ASA catalyst is fully heterogeneous, confirming that there is no leaching in the reaction mixture of any catalytically active species from the catalyst's surface. In the revised manuscript, we have described the results of this experiment on page 10: "To further check the stability and heterogeneity of the catalyst we performed a leaching test, whereby the catalyst was separated from the reaction mixture after 5 min from starting the reaction, after which point no further glycerol conversion was observed by GC, confirming that there is no leaching of any catalytically active species from the catalyst's surface in the reaction mixture." Please also see our reply to comment 2.4.
Comment 2.2. Furthermore, was the reaction monitored over the 1 h time period, is there any significant change in selectivity or reduction in reaction rate over the typical reaction profile seen whereby the rate reduces as the reactants are used up?

Reply:
We have conducted the time resolved study to find the catalyst's activity and glycerol conversion. Glycerol conversion increased from 64.7% (at the first 30 min) to 96.5 % at 60 min, as shown in the figure below. After 120 min of reaction, there is a drop in the glycerol conversion (92.5%) and solketal selectivity (92.1%). Similarly, the reaction rate (specific productivity in this case) is also decreasing (see figure below). The apparent reduction in conversion and is attributed to product hydrolysis by water, which is formed during the reaction ( Figure 4 of the manuscript). The drop in specific productivity (reaction rate), as also the reviewer commented, is attributed to the consumption of the reactants. Comment 2.3. The authors apply the catalyst to another process and highlight its efficacy and helpfully compare the ester yield to other similar catalysts. Clearly, this should inspire other groups to consider this type of material. However, two points strike me as being a challenge when discussing new approaches to industrial application. Complicated catalyst preparations and use of crude feed-stocks. The catalyst is an order greater in activity to H2SO4 and recyclable without the side-waste described by the authors. This element is not a large hurdle and would be worthwhile to pursue following some analysis of the potential waste vs productivity and economics. The later point is perhaps more pertinent, in that typically glycerol is not available cheaply in a purified form. The biodiesel industry produces a very mixed glycerol waste stream. Tolerance of those additives would be a clear advantage and perhaps worth studying in the future.

Reply:
We thank the reviewer for the valuable suggestions. Indeed, the tolerance of catalyst to additives, which appear in industrially produced glycerol is an important aspect and must be the focus of future studies.
Comment 2.4. Could the authors comment on the general robustness of the catalyst, TGA experiments are mentioned and show that the taurine groups are removed >200C which suggests good adhesion? Perhaps this relates to the above points which were not addressed with respect to time-on-line profiles and potential desorption of taurine during reaction. I fully appreciate that the catalyst appears recyclable, however, perhaps the taurine density is such that it can afford to lose an appreciable quantity of active sites and maintain good activity. In summary, I recommend publication following addressing these minor points.
Reply: This point highlighted by the reviewer is very important. To secure that there is no release of taurine units in the reaction medium, we performed the "leaching test". According to this, the catalyst was separated and removed from the reaction mixture after 5 min from starting the reaction. The reaction was allowed to continue. However, glycerol and solketal concentrations did not change further, indicating that the G-ASA catalyst is fully heterogeneous, confirming that there is no leaching in the reaction mixture of any catalytically active species from the catalyst's surface. In the revised manuscript, we have described the results of this experiment on page 10: "To further check the stability and heterogeneity of the catalyst we performed a leaching test, whereby the catalyst was separated from the reaction mixture after 5 min from starting the reaction, after which point no further glycerol conversion was observed by GC, confirming that there is no leaching of any catalytically active species from the catalyst's surface in the reaction mixture." The TGA results indicate as well the strong/covalent bonding of taurine on graphene since the sulfurcontaining gasses are released with a maximum rate above 350 °C ( Figure 1d in the MS) Furthermore, on page 8 of the revised manuscript, it is mentioned that: "The N 1s core level XPS spectrum (Figure 1f) showed three components at BEs of 399, 400.1, and 401.6 eV, assigned to the secondary non-protonated amine (C-NH-C), to the related hydrogen bonding configurations, 56 and the protonated 55 secondary amine groups, respectively. The N 1s XPS core level spectrum of pure taurine ( Figure S2) shows a substantial shift for all three N-components at higher eVs in comparison to G-ASA, indicative of the lower electron density of the primary nitrogen in taurine in comparison to the secondary nitrogen in G-ASA, thus confirming the covalent conjugation of taurine to the graphene support." Finally, elemental analyses with XPS of the used catalyst after the third cycle for the solketal reaction and after the fifth cycle of the esterification reaction showed that the sulfur content remained unchanged, thus verifying its stability.
In conclusion, the described experiments confirm the stability of the catalyst and the robust immobilization of taurine on the graphene skeleton.